1. Field of the Invention
The present invention relates to a gate driver for a transistor bridge circuit and, more particularly, relates to a high voltage gate driver employing current limiting elements for reducing the deleterious effects of voltage spikes resulting from transistor switch commutation into inductive loads.
2. Related Art
Reference is made to FIG. 1 which shows a typical half bridge power conversion circuit employing two series coupled transistors Q1, Q2 connected across a source of high voltage +Hv, -Hv. In this case, the transistors Q1, Q2 are insulated gate bipolar transistors (IGBTs) each including an anti-parallel diode D1, D2, respectively, coupled thereacross. The IGBT Q1 is typically referred to as the "high side" transistor (or switch) and the IGBT Q2 is typically referred to as the "low side" transistor (or switch).
As is the case in practical circuits, an amount of stray inductance, Ls, exists between the series coupled IGBTs Q1, Q2, where Ls may be undesirably introduced due to interconnections within the IGBT Q1, Q2 packages and/or due to printed circuit board runs.
The node U (between the IGBTs Q1, Q2) is coupled to a load (not shown) such that current may be delivered to and received from the load as is known in the art.
As shown, a high voltage driver circuit (or "driver") is used to alternately bias the IGBT Q1 and the IGBT Q2 on and off in response to a control signal (for example, a pulse width modulation signal, not shown). The high voltage driver circuit includes first and second gate driver circuits, Drv1 and Drv2, respectively, for providing bias current to the gates of Q1 and Q2. Gate resistors, Rg1 and Rg2 may be included to insure proper turn on and turn off characteristics of the IGBTs Q1, Q2.
The high voltage gate driver circuit obtains operating voltage from a DC supply, Vcc, where the low side driver, Drv2, obtains operating voltage directly from Vcc and the high side driver, Drv1, obtains operating voltage through a bootstrap circuit. The bootstrap circuit includes a bootstrap diode, Dbs, coupled at its anode to Vcc and at its cathode to one end of a bootstrap capacitor, Cbs as is known. The other end of Cbs is connected to node U. Thus, Drv1 obtains its operating voltage across Cbs.
A shunt resistor Rs may be included between the -Hv node and the Vss terminal of the high voltage gate driver circuit.
The high voltage gate driver circuit may be a "junction isolated" device, which devices include a substrate diode, Dsub, as shown. Junction isolated high voltage gate driver circuits may be obtained from the International Rectifier Corporation (El Segundo, Calif.) under the IR21XX series, IR22XX series and other part numbers. Alternatively, the high voltage gate driver circuit may be a "dielectric isolated" device, which devices do not include a substrate diode. Dielectric isolated high voltage gate driver circuits may also be obtained from the International Rectifier Corporation.
When the half bridge circuit of FIG. 1 drives an inductive load several problems are likely to result. Specifically, when the IGBT Q1 changes state (from biased on to biased off), the current through Q1 (collector to emitter) falls at a rate of -di/dt. Since the load is inductive, the current flowing through the load will freewheel through diode D2. The current in D2 (from anode to cathode) will thus ramp up at a rate of di/dt.
The ramping current (di/dt) in D2 must flow through Ls, which ramping current causes a voltage spike, Vls, to develop across Ls having a polarity shown. Vls may be expressed in terms of the ramping current through D2 as follows: Vls=Ls.multidot.di/dt.
It is noted that Vls may also be induced when a short circuit shut down occurs at a time when Q2 is sinking current from the load. As will be discussed in detail below, the voltage spike, Vls, is undesirable.
Since diode D2 has a substantially constant forward voltage drop, Vd2 (approximately 0.5 to 0.7 volts), the voltage Vs is driven below -Hv in response to the voltage spike Vls. Indeed, Vs may be described by the following equation: Vs=Vd2-Vls (where the magnitude of Vls is typically much greater than Vd2).
When the high voltage gate driver circuit is of the junction isolated type (i.e., includes a substrate diode, Dsub), a voltage spike Vls will tend to induce a current, Isub, to flow through the substrate diode Dsub. Specifically Isub would tend to flow from Ls, through D2, through Dsub, and through Cbs back to Ls. If Isub is sufficiently high, the driver may malfunction (e.g., latch up) which could result in catastrophic circuit failure (i.e., failure of Q1, Q2, and/or the load, etc.). In addition, Isub current flow causes the Cbs voltage (Vbs) to charge higher which could damage the high side driver Drv1 (possibly also causing catastrophic circuit failure).
When the high voltage gate driver circuit is of the junction isolated type or the dielectric isolation type, a voltage spike Vls across Ls will tend to induce extra current flow through the bootstrap diode, Dbs. Specifically, current would tend to flow from Ls, through D2, through Rs, through Vcc, through Dbs, and through Cbs back to Ls. Typically, the current induced by Vls through Dbs is higher than Isub because Vcc and Vls are in and additive configuration. Consequently, Vbs increases which could damage the high side driver Drv1 (possibly also causing catastrophic circuit failure).
Although the circuit shown in FIG. 1 is a half bridge circuit, similar voltage spikes occur in single phase full bridge power circuits, three phase full bridge circuits, high side chopper circuits, and the like.
While the prior art circuit of FIG. 1 is suitable for use with smaller power semiconductor die sizes, such as up to International Rectifier die size 3, it is not satisfactory at higher dies sizes, such as International Rectifier die size 4 and above.
Accordingly, there is a need in the art for a high voltage gate driver circuit configuration which overcomes the disadvantages of the prior art by mitigating against the deleterious effects of a stray inductance voltage spike occurring in a switching power circuit.